The injection of energetic beams composed of neutral hydrogen isotope atoms has for generations been a staple means of heating and driving currents in plasmas in magnetically confined fusion devices. These energetic beams are formed by electrostatically accelerating high currents (on the order of tens of amperes) of either positive or negative ions extracted from a large area plasma source (on the order thousands of square centimeters). The ion beam traverses a neutralizer (i.e. a gas cell in the systems built to date), where a portion of the ions are converted to neutral atoms. The residual ions are magnetically or electrically deflected, while the remaining neutral beam crosses the stray magnetic field of the fusion device to enter the confined plasma, where the atoms are ionized and spiral along the magnetic field, transferring energy and momentum to the bulk plasma through collisions.
One of the primary challenges encountered by these neutral beam systems is holding large voltage gradients across the gaps between the grids and support structures that make up the electrostatic accelerator. Earlier generations of beam systems, accelerating positive ions to 30-140 keV, typically had one or two acceleration gaps, while the higher energies used in negative ion systems required for larger fusion devices typically have several acceleration gaps. Vacuum breakdown between grids at different potentials, and also between their support structures inside the vacuum, limits the potential gradients that may be sustained in such electrostatic accelerators, and requires large amounts of time to be devoted to conditioning the electrostatic accelerators.
Conditioning is a procedure in which the applied voltage is raised in small increments, pausing whenever a breakdown is encountered, in order to allow the energy associated with subsequent breakdowns by micromachining the voltage holding surfaces until they are capable of sustaining such voltages, before moving up in voltage slightly and repeating the process. For the conditioning process to work, the fault energy and maximum current need to be large enough to smooth microprojections that concentrate the electric field, but not so large as to pit the electrode surface and create more emitting points. This process is typically done first without extracting an ion beam, and, then, much more slowly with beam extraction.
In neutral beam systems operated over the past four decades, the beam pulses have had maximum durations of fractions of seconds to tens of seconds, and low duty factors. Conditioning such electrostatic accelerators close to their full design voltage has typically taken many days to some months of operation. The most ambitious operational neutral beam systems, designed to operate at 500 keV, have never accelerated a beam at more than about 410 keV, and almost all of their operations have been at 370 keV or lower, partly because of voltage holding problems and partly because of ion source problems. Negative ion neutral beam systems being developed are supposed to operate at energies of 1 MeV for pulse lengths of up to 1000 seconds with high reliability and infrequent maintenance. Achieving such performance requires that every opportunity to improve the operability of high current large area electrostatic accelerators is explored.